The major thrust of this proposal is the investigation of Cl-distribution in cardiac cells. We now have good, but not conclusive, evidence that Cl- is actively transported into dog cardiac Purkinje fibers and cat myocardium (papillary muscles). We intend, first of all, to do the necessary experiments to conclusively demonstrate that Cl- is actively transported into the above-named cardiac preparations. When these experiments have been concluded, we will proceed to look for specific inhibitors of Cl- transport. We will then use the Cl-inhibitors to manipulate intracellular Cl- activity (aiCl) while looking for the effects of altered aiC1- on electrical activity and regulation of intracellular pH. Throughout these studies, Cl selective microelectrodes will be used to directly measure aiCl. With the help of the recently developed double barrel Cl- microelectrodes, we will extend these studies to cardiac pacemaker cells. In another project we will use double barrel K ion activity (aiK9 in dog cardiac Purkinje fibers exposed to the cardiac glycoside ouabain. An earlier study indicated that the depolarization associated with ouabain toxicity is not the result of decreased aiK. We will try to confirm that observation and, if it is confirmed, look for the cause of the depolarization. The most likely possibilities are extracellular accumulation of K ion or membrane permeability changes. Experiments to test these possiblities are described.